The kidneys filter our blood and get rid of wastes by making urine. Urine leaves the kidneys and passes down into the bladder through tubes called ureters. If the ureters tunnel into the bladder correctly, the urine can only go forwards. If the ureters plug in without a tunnel, urine can back up into the kidneys when the bladder is full or when the child is peeing. If the urine is infected and goes backwards it can cause kidney infections (pyelonephritis) which damages the kidney permanently.